1. Field of the Invention
The present invention relates to a display substrate and a method of manufacturing the display substrate. More particularly, the present invention relates to a display substrate for an electroluminescent display device and a method of manufacturing the display substrate.
2. Description of the Related Art
Nowadays, organic light-emitting diode (OLED) display devices are attracting attention as display devices that can overcome problems of liquid crystal display (LCD) devices. An OLED device includes two electrodes and an organic light-emitting layer disposed between the two electrodes. An electron injected from one electrode and a hole injected from the other electrode are combined with each other in the organic light-emitting layer to form an exciton, and the exciton discharges energy and emits light. The OLED device corresponds to a self-light-emitting device, and does not require a separate light source. Thus, power consumption is low, and display quality, including factors such as response time, viewing angle, contrast ratio, etc., is excellent.
The OLED device of an active matrix type includes a switching thin-film transistor (TFT) connected to a signal line to control a data voltage and a driving TFT receiving the data voltage and applying the data voltage to a control electrode, so that current passes through a light-emitting element.
In order to obtain optimized characteristics of the OLED device, the switching TFT and the driving TFT require different characteristics from each other. The switching TFT requires a high on/off current ratio (Ion/Ioff). In contrast, the driving TFT requires high electric mobility and stability so that sufficient current may pass through a light-emitting element.
Thus, the driving TFT typically employs a polysilicon TFT having high electric mobility. In order to manufacture the polysilicon TFT, a method in which amorphous silicon is heat-treated in a furnace at a high temperature of about 600° C. to about 700° C. to be crystallized into polysilicon is usually used. However, a glass substrate is expanded during the above heat-treatment process, so that misalignment between upper and lower patterns is generated to deteriorate the reliability of a manufacturing process.